Generation of magnetic fields for magnetic resonance imaging (MRI) typically makes use of superconductive coils providing a negligible electric resistance and thus a magnetic field of a required magnitude if supplied with an electric current. Superconductivity is typically achieved in a temperature region at a few Kelvin, e.g. in the range of 4 to 10 Kelvin, which is for instance around the boiling temperature of liquid helium. Once the magnet coil is cooled down to such a superconducting level, a current will continue to flow through the coil due to the negligible coil resistance even after the electric power supply is removed, thereby maintaining a strong, steady magnetic field.
In typical magnetic resonance imaging systems, the main superconducting magnet coils are arranged in a cylindrically shaped pressure vessel, which is contained within an evacuated vessel and forms an imaging bore in the central region. This main magnet coil develops a strong magnetic field in the imaging bore that has to be very homogeneous and temporally constant for accurate imaging.
Superconducting temperatures can be obtained by boiling a liquid cryogen, typically liquid helium within the pressure vessel. However the provision of a steady supply of liquid helium for a MRI system and its storage and use are difficult and costly.
As a consequence, mechanical displacement type cryocoolers for recondensing and recycling the boiled helium gas are commonly used in MRI systems. Cryocoolers might not only be used for recondensing liquid helium gas but also for cooling the superconductive coils directly. A type of cryocooler that is capable of providing a sufficient amount of cooling capacity uses rare-earth materials such as e.g. Er3Ni, HoCu2 or ErNiCo. The moving piston of a two-stage cryocooler is often referred to as displacer. The rare-earth materials are part of the regenerator of the second or cold stage and thus also part of the displacer. The reciprocating movement of the rare-earth materials produces relatively high heat capacity in the superconducting temperature range from 4 to 10 Kelvin because of magnetic transitions and therefore enables low temperature operation. However the rare-earth materials also feature non-negligible magnetic properties. They can be magnetized by the local magnetic field of the superconductive main magnet and thus behave like a moving magnet which can in turn cause magnetic field fluctuations and noise in the imaging volume of the superconductive main magnet. This leads to unacceptable image artifacts like ghosting in the acquired images and as a consequence the cryocooler has often to be switched off during a high-resolution scan process. This complicates the scanning process and diminishes the lifetime of the cryocooler.
The European Patent Application EP 0 955 555 discloses a cryocooler with a superconducting sleeve for a helium recondensing magnetic resonance imager. There, a magnetic superconducting shielding sleeve surrounds a portion of the rare-earth displacer in the terminal portion of the cryocooler housing. The shielding sleeve is in close proximity to the cold head of the cryocooler, is magnetically coupled to the magnet fields generated by moving of the rare-earth displacer, and surrounds 90-270° of the cold head to which it is thermally coupled. Superconducting flow of the currents induced in the shield by the magnetic fields generated by magnetization and movement of the rare-earth displacers oppose the induced magnetic field and shield the MRI imaging volume from the temporal and spatially varying magnetic fields generated by the movement of the rare-earth displacer.
Apart from the relatively high costs of superconductive material it has to be further guaranteed that the superconductive shield is also cooled to the superconductive temperature level.
The present invention therefore aims to provide an improved cryocooler assembly comprising a less cost intensive and more effective ferromagnetic shield for the rare-earth regenerator.